During the assembly of some vehicles it is common to support a vehicle frame on an assembly line with its bottom side up while the suspension system components are attached to the frame, the axle assemblies are attached to the suspension system components, and other bottom side components are attached. The frame is then rotated 180.degree. to place the frame above the axles and in position to receive additional vehicle components.
Machines to rotate vehicle frames are made by Harnischfeger Corporation in Madison Heights, Mich. These machines includes drums that rotate about two parallel generally horizontal axes. A plurality of slings with an eye on each end have a first eye attached to drums that rotate about one drum axis and a second eye attached to drums that rotate about the other drum axis. The center portion of each sling passes under the vehicle frame. At least two slings are employed to rotate each vehicle frame. Additional slings are added as required depending upon the weight of the frames.
Rotation of the drums on both drum axes to take up the slings will raise a vehicle frame. Rotation of the drums on both drum axes to let out the slings will lower a frame. A frame is generally rotated about its long axis by rolling the slings up about one axis while unrolling the slings from the other drum axis. It is also possible to rotate a frame about the long axis of the frame by rolling the slings up or unrolling the slings from one drum axis while holding the drums on the other drum axis from rotating. However, when the drums on one axis are rotated and the drums on the other axis are held in a fixed position, the slings have to be longer and frame rotation is slower.
The vehicle frames tend to slide on the slings during frame rotation. The frames also tend to swing from side to side when supported by the slings. The sliding and swinging tends to abraid and fray nylon webbing used to make the sling. The fraying and abrading occurs where the slings contact the vehicle frame as well as where the eyes of the slings are attached to the drums. The slings are replaced when there is some abrading and fraying because sling failures cannot be permitted on an assembly line where people are working.
Slings made from unprotected nylon webbing last about two hours on an assembly line running at the rate of 70 units per hour. This short sling life makes operation of the vehicle frame turnover machine expensive.